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Biomedical subjects

J T Chan

Publications and source records attributed to J T Chan.

At least 37 records · Page 2Linked to original sources
PubMed1993

Tooth enamel dust as an asthma stimulus. A case report.

A case report of a first-year dental student with asthma, who experienced exacerbation of symptoms and a severe asthmatic crisis in the course of her preclinical dental training, is presented. Dust generated as a result of preparing natural teeth triggered the bronchoconstrictive response. Her subsequent medical and preventive measures are cited. This case identifies, for the first time, enamel dust as an asthma stimulus, thus serving as a precaution to prospective dental students and personnel afflicted with the disease and emphasizing the importance of effective face masks in dental laboratories during dust-generating procedures.

Adult↗
PubMed1993

Fluctuations in tap water fluoride levels: a potential problem for practitioners.

Weekly water samples were obtained from the tap water at the Dental Branch of the University of Texas Health Science Center at Houston. Analysis of these water samples showed significant fluctuations in fluoride levels from 1988 to 1991. This inconsistency in water fluoride content makes it very difficult for health care providers to prescribe and maintain an optimal fluoride supplementation to children. Based on results from this study, more rigorous surveillance and monitoring of water fluoridation in Houston is recommended.

Fluoridation↗
PubMed1990

The role of caffeinated beverages in dental fluorosis.

Recent studies have demonstrated that the incidence of dental fluorosis has increased during the past decade. Greater availability and use of fluoride-containing gels, mouth rinses, dentifrices, etc., improper prescribing of fluoride supplements and ingestion of fluoride dentifrice by some children are some of the suggested determinants of dental fluorosis. However, based on the increase in consumption of tea, coffee, and other caffeine-containing beverages by the children, and the augmentative effect of caffeine on fluoride bioavailability, we theorize that the rise in incidence of dental fluorosis in North America is mainly due to the replacement of water intake by caffeine-containing beverages among the young population.

Adolescent↗
PubMed1990

Relationship between plasma and tear fluoride levels in rabbit and man.

Tear fluoride concentrations were determined in rabbits and man. Tear fluoride levels in reflex tears were higher in rabbits (2-10 mumol/l) than in man (1.4-4 mumol/l), while plasma levels were quite similar in both species (1.5-4 mumol/l). Chronic fluoride ingestion in rabbits over 29 days of 1, 5, 10, 25 or 50 (2.65 mmol/l) ppm fluoride in drinking water caused a dose-related increase in plasma, but not tear, levels of fluoride. The plasma:tear ratio went from 0.5 at baseline (1 ppm) to 1.8 after drinking water that contained 50 ppm F. Acute fluoride ingestion in man led to an increase in both plasma and tear fluoride levels that changed such that the plasma:tear ratio remained constant despite a 2-fold increase in plasma fluoride.

Animals↗
PubMed1990

Influence of coffee on fluoride metabolism in rats.

Concomitant intragastric administration of sodium fluoride and coffee resulted in a significantly higher (P less than 0.01) plasma fluoride level than intake of the same amount of fluoride with water. The same result was obtained when coffee was substituted with an equivalent amount of caffeine. Comparison of plasma fluoride levels by total area under the curve of plasma fluoride concentration versus time indicated an almost 2-fold difference. Although the mechanism(s) is not known, it appears that caffeine is responsible for the present observation. This finding could help explain the variations in the incidence of dental fluorosis among people living in optimally fluoridated communities.

Animals↗
PubMed1990

Fluoride content of bottled waters: implications for dietary fluoride supplementation.

Increase in the popularity of bottled water in the Metropolitan Houston area prompted this investigation about the fluoride content of bottled waters, since bottled water likely substitutes tap water for drinking water used by adults as well as infants in many households. Results from this study indicate that the fluoride content of most of the bottled waters are well below 0.3 ppm, a level at which dietary fluoride supplementation is recommended even for infants under two years of age. The importance of knowledge of water fluoride content in dietary fluoride prescription for pediatric patients is discussed.

Fluorides↗
PubMed1989

The distribution of fluoride of prenatal origin in the rat--a pilot study.

Sixteen female, Sprague-Dawley rats were divided into four equal groups. Two groups served as controls receiving low or regular concentrations of fluoride (F); animals in the other two groups were given drinking water, containing 100 parts/10(6) F, for 3 weeks either during or immediately before pregnancy. Thirteen days after delivery, the pups and dams were killed and various tissues analysed for F content. Prenatal F supplementation increased F concentrations in plasma, mandibular incisors and femoral epiphyses of pups by 25, 36 and 38% respectively, when given during pregnancy. Only a slight increase of 9 and 11% in the respective F concentrations of incisors and epiphyses occurred when the supplement was given before pregnancy. The fluoride level of milk was consistently higher than that of the maternal plasma. These results suggest the need for further study of prenatal F supplementation.

Animals↗
PubMed1980

Low molecular weight peptidic fraction in the chromatin from normal and cancer cells: control of transcription.

DNA isolated from cell nuclei by intensive deproteinization with chloroform/isoamyl alcohol and phenol extractons contains a low molecular weight peptidic fraction in a quantity of about 20 micrograms/mg DNA. These peptides were characterized by chromatography on CM-Sephadex, Sephadex G-25, high performance liquid chromatography on microBondapak C18 and amino acid composition. The peptides control transcription in a reconstituted cell-free system with prokaryotic and eukaryotic RNA polymerase and stabilize the structure of double stranded DNA, while increasing its melting point. Their level is markedly decreased (by about 40%) in DNA prepared from tumor cells as compared to normal cell DNA. Transcriptional studies showed only a slightly increased template activity of DNA extracted at pH 9.5 versus DNA extracted at pH 6.0 for DNA preparations from tumor cells. However, there was a marked increase in template activity for DNA preparations treated at pH 9.5 from normal cells--232%, 124%, 97% and 78% for rat liver, mouse liver, mouse thymus and fibroblast L-929 cells, respectively. Also there was no difference in the melting point between these two preparations of DNA from tumor cells; normal cell DNA preparations showed increased melting point of preparations treated at pH 6.5. The data obtained indicate that the loss of low molecular weight peptides from tumor DNA during carcinogenesis is responsible for uncontrolled gene expression observed in cancer.

Animals↗
PubMed1979

Sterol synthesis: studies of the metabolism of 14 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol.

[3 alpha-3H]14 alpha-Methyl-5 alpha-cholest-7-en-3 beta-ol has been prepared by chemical synthesis. The metabolism of this compound has been studied in the 10,000 g supernatant fraction of liver homogenates of female rats. Efficient conversion to cholesterol was observed. Other labeled compounds recovered after incubation of [3 alpha-3H]14 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol with the enzyme preparations include the unreacted substrate, 5 alpha-cholesta-7,14-dien-3 beta-ol, 5 alpha-cholesta-8,14-dien-3 beta-ol, cholesta-5,7-dien-3 beta-ol, 5 alpha-cholest-8(14)-en-3 beta-ol, 5 alpha-cholest-8-en-3 beta-ol, and 5 alpha-cholest-7-en-3 beta-ol. In addition, significant amounts of incubated radioactivity were recovered in steryl esters. The steroidal components of these esters were found to contain labeled 14 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol, 5 alpha-cholesta-8,14-dien-3 beta-ol, 5 alpha-cholesta-7,14-dien-3 beta-ol, 5 alpha-cholest-8-en-3 beta-ol, 5 alpha-cholest-7-en-3 beta-ol, and cholesterol.

Animals↗
PubMed1978

The mitigating effect of dietary antioxidants on chemically-induced carcinogenesis.

The effect of a dietary antioxidant mixture on 3-methylcholanthrene mediated carcinogenesis in hairless mice was investigated. The antioxidant mixture significantly reduced the frequency of premalignant lesions and their subsequent development into tumors. The similarities in response of chemical and UV light-carcinogenesis to these antioxidants suggest some congruity in the mechanisms of the carcinogenic process.

Animals↗
PubMed1978

Systemic protection by antioxidants against UVL-induced erythema.

An antioxidant supplemented diet provided marked systemic protection against ultraviolet light mediated erythema in hairless mice. Among the individual constituents of the diet, butylated hydroxytoluene was most effective whereas glutathione and vitamins C and E afforded negligible protection. The mixture of antioxidants, and butylated hydroxytoluene individually, demonstrated diminished, but significant, protection when applied topically. The safety of this systemic photoprotectant and its clinical relevance at present is unknown.

Administration, Oral↗
PubMed1978

Effects of dietary constituents on ultraviolet light-mediated carcinogenesis.

The effects of several dietary supplements of antioxidants and enzyme inducers on ultraviolet light-mediated carcinogenesis were investigated. Glutathione (reduced) was without effect, but butylated hydroxytoluene, phenobarbital, and disulfiram all significantly suppressed the initiation and development of actinic lesions and tumors. On the basis of the present study and related previous ones, tumor inhibition appears to be due not to an umbrageous effect but rather to the induction of systemic physiological responses.

Animals↗
PubMed1977

Physiological changes in hairless mice maintained on an antioxidant supplemented diet.

Several physiological parameters were measured in hairless mice maintained on a diet supplemented with antioxidants. Comparisons to animals on control diet revealed higher water-soluble antioxidant content of skin and increased liver weight. Only small differences in body weight occurred and no distinct histological changes were observed in skin or liver.

Animals↗
PubMed1977

Antioxidant-mediated reversal of ultraviolet light cytotoxicity.

Several antioxidants were tested for their protective effect in Chinese hamster embryo cell against ultraviolet light (UVL)-irradiation. Ascorbic acid, DL-alpha-tocopherol, butylated hydroxytoluene, and reduced glutathione were all shown to reverse UVL-induced cytotoxicity. At concentrations tested, no protective effect was detected when any of these antioxidants were added prior to UVL irradiation.

Animals↗